The present invention relates to a calibration apparatus applicable to a TDMA (Time Division Multiple Access) based and a CDMA (Code Division Multiple Access) based digital radio communication.
In conventional digital radio communications, a multiple access system is used and there are cases where an adaptive array antenna is used as the antenna. The multiple access system refers to a channel access system when a plurality of stations perform communications simultaneously using a same band. The TDMA system included in this multiple access system is called xe2x80x9ctime division multiple access system.xe2x80x9d This TDMA system implements multiple accesses by allowing a plurality of stations to use carriers of the same frequency, converting signals transmitted from those stations to intermittent signals (here, referred to as xe2x80x9cburst signalsxe2x80x9d) and aligning the burst signals of those stations in such a way that they do not collide with each other on the time scale.
However, the TDMA system has the difficulty in fully suppressing interference with other stations and this originates various problems such as increasing the number of interference signals as the number of multiplexing stations increases, making it difficult to acquire synchronization, deteriorating the communication quality and making communications impossible. If interference with other stations described above can be fully suppressed, it will be possible to improve the frequency utilization efficiency, improve the communication quality of each station in the same cell (area) and increase its capacity (multiplexing number or the number of channel accesses).
On the other hand, the adaptive array antenna is a system that determines a weight of each antenna output based on a control algorithm and controls directivity according to changes in the surrounding conditions. In the array antenna made up of a plurality of antennas, combining antenna outputs with an amplitude/phase shift added changes array directivity.
This adaptive array antenna is explained with reference to FIG. 18. FIG. 18 shows an overall configuration of a reception adaptive array antenna. In FIG. 18, antenna outputs 1802 from a plurality of antennas 1801 are multiplied by weights 1803 and combined into array output 1804. Here, weights are controlled by weight control section 1807 based on the following 3 pieces of information:
{circle around (1)} Combined array output (1805)
{circle around (2)} Each antenna output (1802)
{circle around (3)} Advance knowledge of desired signal (1806)
There are also cases where combined array output (1805) is not used for weight control.
Conventionally, the adaptive array antenna has been researched and developed as an antenna system to maximize SINR (Signal to Interference plus Noise Ratio) of a reception signal. The adaptive array antenna is also used as a countermeasure against interference among different stations in TDMA transmission. This adaptive array antenna in TDMA transmission is explained with reference to FIG. 19.
FIG. 19 shows an overall configuration of a TDMA reception adaptive array. In FIG. 19, reception outputs 1903 from radio sections 1902 connected to a plurality of antennas 1901 are multiplied by weights 1904 and combined into array output 1905. Weight control is performed in the same way as the control in FIG. 18 above. Reception data 1906 is obtained from array output 1905.
FIG. 20 is a conceptual diagram of TDMA transmission using an adaptive array antenna on the receiving side. Suppose BS 2001 is provided with a reception adaptive array antenna and is communicating with first MS 2002 equipped with a non-array antenna. At this time, BS 2001 eliminates delayed signals (2003 and 2004) by controlling directivity and suppresses interference signal from another station, second MS 2005, using the same frequency.
However, in FIG. 19, the amount of variation (D1, D2, . . . , Dn) made up of phase variation and amplitude variation generally varies among different radio sections 1902 due to variations in the delay characteristics and amplitude characteristics of elements such as amplifier and filter. Therefore, different phase variations and amplitude variations are added in different radio sections 1902 and the phase and amplitude of the reception signal at the antenna reception end and the phase and amplitude of the input signal to weight control section vary from one antenna to another. Because of this, the directivity pattern including a null point obtained from a weight convergence result is different from the actual directivity pattern.
Furthermore, when transmission directivity is controlled using the reception weights above, correct directivity control is not possible. To prevent the phenomena above, it is indispensable to retain the phase difference and amplitude ratio of the reception signal at each antenna reception end in the stage of signal input to weight control section 1907, too. To do this, it is necessary to detect the delay (D1, D2, . . . , Dn) and amplitude of each radio section beforehand and compensate the variations (differences) of the amount of delay and amount of amplitude using some method.
One possible compensation method is the method of multiplying reception outputs 1903 from the radio sections in FIG. 19 by phase offsets corresponding to the delay difference and gain offsets corresponding to the amplitude ratio. Regarding detection of variations in the phase and amplitude characteristics of an adaptive array apparatus, there is a report in the thesis G. V. Tsoulos, M. A. Beach xe2x80x9cCalibration and Linearity issues for an Adaptive Antenna Systemxe2x80x9d (IEEE VTC, Phoenix, pp.1597-1660, May 1997). The thesis above proposes a system using a tone signal as the calibration signal.
A calibration apparatus of radio sections in conventional TDMA transmission using this tone signal is explained with reference to FIG. 21. FIG. 21 is a block diagram showing an overall configuration of the calibration apparatus in the conventional radio section. FIG. 21 illustrates a case where the number of antennas is 2.
Tone signal (sine wave signal) 2102 generated from calibration signal generator 2101 is input to radio transmission section 2103. In this example, the reception sections perform quadrature modulation and sin(xcfx89t) and cos(xcfx89t) are input as orthogonal IQ signals. Suppose tone signal cycle T at this time is 2xcfx80/xcfx89 and for information symbol frequency fs, xcfx89=fs/m (m greater than 1). FIG. 22 shows a constellation of the tone signal in the IQ plane. The signal rotates on the circumference in the figure with a constant cycle of 2xcfx80/xcfx89.
Radio transmission section 2103 has a function of transmitting signals with reception carrier frequency fc of the radio reception sections that carry out delay detection. The signal output with carrier frequency fc is sent via a cable, etc. from transmission terminal 2104 to antenna connection terminals 2107 and 2108 of radio reception sections 2105 and 2106, respectively. At this time, suppose these cables are equal in length with sufficient accuracy relative to the wavelength of the carrier frequency. Quadrature detection outputs 2109 and 2110 of their respective radio reception sections are input to detection circuit 2111. Detection circuit 2111 compares input tone signal 2102 and detection output 2109 and detects:
(Amplitude ratio, phase difference)=(Ar1, xcex94xcfx86r1) (2112)
Detection circuit 2111 also compares tone signal 2102 and detection output 2110 and detects:
(Amplitude ratio, phase difference)=(Ar2, xcex94xcfx86r2) (2113)
FIG. 23 is a constellation example of tone signal a(t) and detection output b(t) at time t. At this time, the relationship between b(t) and a(t) is expressed using phase difference xcfx89 and amplitude ratio A as follows:
b(t)=Axc2x7exp(jxcfx86)xc2x7a(t)
Here, phase difference xcfx86 represents a delay (amount of phase) of the remainder (Dmodxcex: mod is remainder operator) obtained by dividing total delay D of delay Dt of the radio transmission section, cable delay Dk and delay Dr of the radio reception section (D=Dt+Dk+Dr) by tone signal wavelength xcex=c/xcfx89 (c: velocity of light).
In FIG. 21, since delay Dt of radio transmission section 2103 and cable delay Dk are common to two radio reception sections 2105 and 2106, the difference between detected phase differences xcex94xcfx86r1 and xcex94xcfx86r2 is the difference in delay between radio reception sections 2105 and 2106. Moreover, amplitude ratio A represents the ratio of the amplitude of calibration signal 2102 to the amplitude of the detection output. Therefore, the ratio of detected amplitude ratio Ar1 to Ar2 represents the difference (amplitude ratio) in the amplitude characteristic between reception sections 2105 and 2106.
Detecting the amplitude ratio and phase difference of each radio section using the above apparatus beforehand can compensate variations (differences).
However, since the calibration signal of the calibration apparatus of the radio sections in the conventional TDMA transmission is a tone signal, only the delay characteristic and amplitude characteristic at a specific frequency, for example central frequency f0, are measured. However, a TDMA transmission modulated signal used for actual communications is a wideband signal and group delay characteristic and frequency characteristic of a filter, etc. in the reception section vary in delay and attenuation depending on the frequency.
Therefore, the calibration apparatus in the conventional TDMA transmission cannot measure the delay characteristic and amplitude characteristic at the reception sections accurately when a demodulated signal is received.
FIG. 24 is a drawing showing the spectrum status of a calibration signal and transmission signal. In FIG. 24, while the modulated signal is a wideband signal with bandwidth M [Hz] centered on central frequency f0, the calibration signal is a line spectrum. In this way, the calibration signal is by far different from the actual modulated signal.
It is an objective of the present invention to provide a calibration apparatus capable of accurately detecting the delay characteristic and amplitude characteristic of radio reception sections and radio transmission sections in transmission.
This objective is achieved by a calibration apparatus that detects delay characteristic and amplitude characteristic of radio reception sections using a calibration signal of the same band as or a band close to that of a modulated signal used in actual communications.